In the general construction of a water heater, a heating element is provided to heat the water within the water heater vessel and a separate thermostatically controlled device is provided to maintain the water at a predetermined temperature. In such devices, it is also necessary to ensure that the level of water in the vessel is covering the heating element whenever the element is heating. If water is not covering the element, failure of the element eventually occurs. Thermostatic control devices are generally not able to safeguard against element failure in these "boil dry" situations and a separate override facility is required to turn off the heating element.
In addition, it is desirable to control the operation of a water heater using an integrated circuit controller which operates the heating element through a triac. However solid state devices generally fail at temperatures approaching 120.degree. C. leaving little margin between the operating temperature of water heaters, when operating near the boiling point of water and the failure temperature of these devices.
When designing a water heater to operate at a temperature near the boiling point of water, there are a number of possible conditions each having associated problems which must be overcome for the water heater to operate effectively and reliably.
During the heating of the water, while the vessel is full, the electrical power is provided to the heating element continuously. Electronic switching devices such as a triac which switch the power to the element, generate heat at a rate in proportion to the current flowing in the element. When the operating temperature is near the boiling point of water, this heat needs to be dissipated efficiently to ensure the device does not over heat.
Commonly used triacs control the flow of current with an integrated controller which are a few millimeters square and about 0.5 mm thick. The maximum temperature this controller can tolerate is 120.degree. C. If the integrated controller overheats, the triac loses its ability to control the flow of current and remains in the "on" position permanently. When the triac is in a latched "on" position, the water in the water heater boils away causing a separate over temperature protection device to fuse, permanently opening the circuit. When the water is almost at say 95.degree. C., the element is still on full heating duty cycle and thus has the maximum current flowing through the element. Hence the heat produced by the triac must be dissipated to the water to prevent the triac reaching its maximum permitted temperature.
Another condition of water heater operation which must be controlled is when an operator switches on the power to the heating element when the heating element is not covered with water. With no water, the element quickly heats up to red heat and will self-destruct if no protective action is taken to prevent this. An extremely hot element is also dangerous if accidentally touched and if in contact with flammable material causes a fire hazard.